In a vehicular application an anti-theft or security system's main operative power is ordinarily drawn from a vehicle battery and, when operative, the vehicle's associated charging system. To insure security system operation when the main operative power is not available from either the vehicle battery or charging system, a backup battery is typically used. Thus, the backup or reserve battery powers the security system when main operative power is turned off, or cut off.
To insure maximum operating time for the security system, the backup battery is typically a secondary or rechargeable type battery. Optimally, the backup battery will only be charged when the vehicle's charging system is active--so as not to unnecessarily deplete the vehicle battery for the purpose of replenishing the backup battery. Also, in accordance with achieving maximum operating time, the backup battery should only power the security system while the vehicle's charging system and vehicle battery are incapable of doing so.
In a security system it is also vital to minimize the physical evidence of interconnections between the vehicle's electrical system and the security system. This is important to help hide the security system from a thief who may attempt to disable the security system to acquire the vehicle. Optimally, the security system will have minimal external connections, and where external connections are necessary they are preferably localized proximate the security system so that they are easily concealed and not exposed to the thief.
In addition to an electrical connection for providing operative power to the security system from the vehicle battery, prior art schemes have required an additional ignition keyswitch input to determine whether or not the vehicle's charging system is active--and thus when to charge the backup battery. This is not desirable because of the additional wiring exposed to the thief. Furthermore the additional wiring is undesirable because of added manufacturing cost, installation difficulty, and field reliability concerns.
Another requirement in a backup battery management scheme is that the backup battery should not be charged from a voltage source higher than a safe magnitude. If charging is allowed from a voltage source higher than a safe magnitude, then the backup battery may become damaged. Furthermore, if a backup battery is shorted, a backup battery apparatus associated with the prior art backup battery management scheme can be damaged because of excess power dissipation.
What is needed is an improved backup battery management approach that has minimal external electrical interconnections localized where possible, efficiently replenishes a backup battery only while an associated charging system is actively charging a vehicle battery, uses the backup battery to power a security system only while a vehicle's charging system and vehicle battery are incapable of doing so, and limits charging of the backup battery while the charging system is outputting too high a voltage or the backup battery is electrically shorted.